Power conversion devices, such as a device for driving an alternate current motor (so-called inverter) and an Uninterruptible Power Supply (UPS), convert power by switching a power semiconductor element. FIG. 4 shows an exemplary circuit diagram of such a power conversion device. Such a device has a direct current (DC) power source 1, semiconductor power modules 2U, 2V, 2W, each formed of two serially connected Insulated Gate Bi-polar Transistors (IGBT) (acting a power semiconductor element) connected to two Free Wheel Diodes pairs in an antiparallel manner, and a load 3, such as a motor. The semiconductor power modules 2U, 2V, and 2W are connected in parallel, and the upper and lower arms of the IGBT for each phase are alternately switched to convert the direct current of the DC power source 1 to the alternate current, thereby supplying the converted power to the motor 3. The gate driving circuit of the IGBT is not illustrated in FIG. 4.
In a well-known Pulse Width Modulation control method (PWM) for switching an IGBT, a comparison calculation section 5c of an external control circuit 5 compares the magnitudes of a reference sinusoidal wave 5a and an output voltage order 5b to determine a width of a switching pulse. This switching pulse is sent to a driving circuit 4 for conversion to a gate signal to the IGBT, and is output to the power modules 2U, 2V, and 2W, respectively.
The power semiconductor element, including the IGBTs and Free Wheel Diodes, is mounted in a single package to constitute the semiconductor power modules 2U, 2V, and 2W, thereby providing a device of a simple structure. This simplifies the assembly operation and wiring operation, and further allows the elements to be cooled easily.
Recently, another type of power module has been developed. Here, the power semiconductor elements and the driving circuit 4 for driving IGBT are collectively mounted in a single package or container. This kind of module is called an Intelligent Power Module (IPM), the design of which centers on a simplification of the structure of an electric power conversion device. FIG. 5 illustrates such a power conversion device comprising the IPM 2A, which further includes an IGBT, Free Wheel Diodes, a diode, a driving circuit 4 or the like, DC input terminals 2a and 2b connected to a DC power source 1, alternate current (AC) output terminals 2c, 2d, and 2e connected to a load 3, a connection terminal 2f for a braking resistor 11, and a control input terminal 2g connected to a control circuit 5.
In the power conversion device, including the above-described conventional semiconductor power modules 2U, 2V, and 2W and/or IPM 2A, switching of the power semiconductor element causes an excessive switching noise, which can increase the risk of a failure, such as a malfunction of separate devices located around the power conversion device, or undesirable noise to be contained within circuits of such separate devices. The switching noise can be classified into two main types.
The first is a normal mode noise caused by a normal mode high frequency current flowing in a closed loop consisting of a power module and a DC power source. FIG. 6A shows a closed loop 10N in which a normal mode noise current flows. In FIG. 6A, reference numeral 2 denotes a power module representing one phase power module in the above-described semiconductor power modules 2U, 2V, and 2W and the IPM 2A. This normal mode is a mode in which a high-frequency noise current, caused by LC resonance, flows in the closed loop 10N. Specifically, this LC resonance is created, during switching of the power semiconductor element, due to the floating inductance of the wiring constituting the closed loop 10N, and the junction capacitance of the power semiconductor element.
The second switching noise is a common mode noise caused by a noise current flowing the grounding through the floating capacitance in the power module and the power conversion device. FIG. 7A shows the closed loop 10C through which this common mode noise current flows. This common mode is a mode in which a significant voltage change due to the switching of the power semiconductor element (dV/dt) causes the floating capacitances 7 and 8 to be charged and discharged at a high frequency. The charged and discharged currents flow in the closed loop 10C via the grounding. In this mode, the noise current can flow out to the DC power source 1 or emitted as an electric wave.
To suppress the above-described normal mode noise current or common mode noise current, a conventional practiced is to add impedance, such as an inductance, to the loop of a noise current. FIG. 6B shows a conventional technique for suppressing the normal mode noise current by serially connecting the power module 2 to an inductor 6a. FIG. 7B shows a conventional technique for suppressing the common mode noise current by connecting a positive pole and a negative pole of the DC input terminal of the power module 2 to both ends of the DC power source 1 via an inductor 6b. This technique suppresses noise current because the inductor 6b functions as an impedance component to the noise current flowing to the grounding side via the floating capacitances 7 and 8 in FIG. 7A.
In the techniques illustrated in FIG. 6B and FIG. 7B, the normal mode noise current and the common mode noise current can be suppressed to a limited level. In this case, providing the inductor for suppressing the noise current at a position closer to the semiconductor element is effective in downsizing the entire circuit, and in packaging of the entire circuit. In this regard, the techniques described in the following patent publications have been contemplated.
Japanese Laid-Open Publication No. 2000-58740 discloses means for suppressing the common mode noise current. Specifically, the periphery of the positive and negative poles of the inner lead (connection line) for supplying a DC power source of a semiconductor device (IC chip) connected to the DC power source are surrounded by an annular composite magnetic material, which is used as a filter element, thereby filling this material into the package. On the other hand, Japanese Laid-Open Publication No. 9-121016 discloses a noise reduction element made of an amorphous magnetic alloy that is externally fitted with a lead section of various semiconductor elements (e.g., diode) or that is integrally molded.
In the conventional technique described in the former patent publication, the composite magnetic material cannot be changed with a different one (i.e. change the inductance value) because the composite magnetic material used as a filter element is sealed in the package. The frequency of switching noise differs depending on the conditions of an external circuit (e.g., floating inductance of a wiring, floating capacitance). The present inventors have found that it is highly desirable to attach the filter element to the exterior of the package so that the filter element can be changed as required. Such an exchange operation is impossible according to the technique disclosed in the former patent publication. Another problem is that the composite magnetic material sealed in the package enlarges the package. Furthermore, the conventional technique described in the latter patent publication cannot be directly used for a power module having a medium to large capacity since this technique is for a semiconductor element with a lead wiring having a relatively small capacity. It is also impossible to externally fit the noise reduction element as described in the latter patent publication to the terminal of the power module because the terminal is generally connected to a large-sized wiring component (e.g., copper bar having a large area) for larger currents.
Accordingly, there is a need for a semiconductor power module that allows a magnetic member used as a filter element to be easily changeable. Also, there is a need for a semiconductor power module having a smaller package. Moreover, there is a need for a semiconductor power module that can be used regardless of the capacity of each power semiconductor element or the entirety of the module and regardless of the type of Intelligent Power Module, including therein semiconductor power elements for one phase of upper and lower arms and a driving circuit. The present invention addresses these needs.